Illumination system for use in image projection apparatus

ABSTRACT

An illumination system for use in an image projection apparatus, having a light source assembly which includes a lamp and a concave reflector for producing convergent light, and an illumination lens group which includes, in order from the light source assembly side, a first lens unit of a negative power movable along the optical axis, a second lens unit of a positive power and a third lens unit of a positive power. The illumination lens group fulfills the following equation, 
     
         φ1+φ2&lt;0 
    
     where φ1 and φ2 represent the powers of said first and second lens units, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to illumination systems, and moreparticularly to an illumination system for use in image projectionapparatus such as microfilm readers, microfilm reader-printers andoverhead projectors.

2. Description of the Related Art

FIG. 1A shows a source assembly 1 which comprises a lamp 2 and anellipsoidal reflector 3 disposed behind the lamp 2 and which isgenerally used in recent years in image projection apparatus such asmicrofilm readers, microfilm reader-printers and overhead projector.With illumination systems having such a light source 1, the lightreflected from the ellipsoidal reflector 3 contributes to theillumination of an original more efficiently than the light travelingfrom the lamp 2 directly toward the original. As shown in FIG. 1B, thefilament 4 of the lamp 2 is disposed at the position of first focus ofthe ellipsoidal reflector 3, and the light emitted by the filament 4 isreflected at the reflector 3 and concentrated on the position 5 ofsecond focus of the reflector. Use of the ellipsoidal reflector 3 havingthe light concentrating or focusing action results in the advantage thatthe condenser lens system subsequently disposed can be simplified. Suchellipsoidal reflectors include one having a perfectly ellipsoidalreflecting surface, and one having a reflecting surface which isbasically an ellipsoidal surface and formed by a collection of minuteplanes.

FIGS. 2A and 2B show a conventional illumination system including alight source assembly 1 having an ellipsoidal reflector 3(Reader-Printer MFB1100, product of MINNESOTA MINING AND MANUFACTURINGCO.). FIG. 2A shows the system wherein a projection lens 6L of lowmagnification is used, and FIG. 2B shows the system wherein a projectionlens 6H of high magnification is used. A filament 4 emits light 7, whichis reflected from the ellipsoidal reflector 3 and illuminates amicrofilm F held between a pair of holders 9A and 9B. An image of theilluminated microfilm F is projected on a screen (not shown) by theprojection lens 6L or 6H.

The light source assembly 1 of the illumination system is basically sodesigned that the lamp 2 will not block the light from ellipsoidalreflector 3. However, the direct light from the lamp 2 does notsubstantially contribute to the illumination, for example, owing to thescattering by the top end 2A of the bulb, while the bottom portion ofthe reflector 3 is not utilized as the reflecting surface because of thesetting of the lamp 2, so that a bundle of rays illuminating a point Pon the microfilm F has a dark central portion Q and a bright peripheralportion R (see FIGS. 3A and 3B). This phenomenon becomes more pronouncedas the f-number of the projection lens increases on the microfilm sideto result in an uneven illuminance distribution on the screen. Thisproblem can be solved by making the width D of the bundle of rays on theaxis at the right source assembly 1 sufficiently greater than the widthd of the central portion Q. This can be realized by inserting a lens ofsuitable negative refracting power in the bundle of converging rays inthe vicinity of the light source assembly 1. For this purpose, thecondenser lens system 8 used comprises a negative lens 8L positioned inthe vicinity of the light source assembly 1, and a positive Fresnel lens8F positioned in the vicinity of the microfilm F.

For the illumination system to achieve an improved illuminationefficeincy, on the other hand, the system employs Kohler illuminationfor forming an image of the filament 4 at the pupil position Ent. P ofprojection lens 6L or 6H on the film F side. Nevertheless, since theprojection lenses 6L and 6H differ in the pupil position Ent. P in thefocused state, there is a need to alter the position where the image ofthe filament 4 is to be formed, in accordance with which of the lenses6L and 6H is used. With the illumination system described, the Fresnellens 8F close to the microfilm F is therefore shifted axially of thesystem to thereby alter the position where the image of the filament 4is to be formed.

However, the prior-art system requires a space for shifting the Fresnellens 8F which is relatively large and positioned close to the microfilmF, making it difficult to provide a compacted image projectionapparatus. Moreover, a large and complex mechanism is needed forshifting the Fresnel lens 8F.

SUMMARY OF THE INVENTION

Accordingly, the main object of the present invention is to provide anillumination system which is adapted to make an image projectionapparatus compacted when incorporated therein.

Another object of the present invention is to provide an illuminationsystem adapted to afford a uniform illuminance distribution at alltimes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects or features of the present invention will becomeaparent from the following description of a preferred embodiment thereoftaken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B are a perspective view and a sectional view,respectively, showing the construction of a light source assembly incommon illumination systems;

FIGS. 2A and 2B are sectional views showing the construction of aconventional illumination system;

FIGS. 3A and 3B are diagrams illustrating the problem encountered withthe light source assembly shown in FIGS. 1A and 1B;

FIGS. 4A and 4B are sectional views showing the construction of anillumination system embodying the invention; and

FIG. 5 is a diagram for illustrating the radius of curvature of aFresnel surface.

In the following description, like parts are designated by likereference numbers throughout the several drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below withreference to the drawings.

FIGS. 4A and 4B are views showing the construction of an illuminationsystem embodying the present invention. FIG. 4A shows a projection lens6L of low magnification as used in the system, while FIG. 4B shows aprojection lens 6H of high magnification as included in the system.

Light 7 from a filament 4 of a light source assembly 1 is reflected froman ellipsoidal reflector 3 and impinges on a condenser lens system C.The condenser lens system C comprises, as arranged from the light sourceside toward the projection lens, a first lens unit C1 of negative power,a second lens unit C2 of positive power, and a third lens unit C3comprising a Fresnel lens of positive power. A microfilm F held betweenholders 9A and 9B is illuminated with the light through the condenserlens system C, and an image of the microfilm F is projected on a screenby the projection lens 6L or 6H. Furhter to obtain a sufficientilluminance on the screen, the condenser lens system C forms an image ofthe filament 4 at the pupil position Ent. P of the projection lens 6L or6H on the film side.

To form the image of the filament 4 at the pupil position Ent. P at alltimes regardless of the type of projection lens, the first lens unit C1of the condenser lens unit C is movable in the direction of the opticalaxis and is positioned in place for the particular projection lens 6L or6H used. More specifically, the first lens unit C1 is positioned in thevicinity of the second lens unit C2 when the projection lens 6L of lowmagnification is used (see FIG. 4A), or is positioned as shifted towardthe light source assembly 1 when the projection lens 6H of highmagnification is used (see FIG. 4B).

Assuming that the first lens unit C1 and the second lens unit C2 are φ1and φ2, respectively, in refracting power, the condenser lens system Csatisfies the following conditional inequality (A).

    φ1+φ2<0                                            (A)

The inequality (A) represents the requirement for giving a uniformilluminance distribution on the screen and reducing the amount of shiftof the first lens unit C1 in the case where the projection lens of lowmagnification is used. The reduction in the amount of shift of thesecond lens unit C2 serves to make the shifting mechanism thereforcompact.

Each of the lens units C1, C2 and C3 may comprise a single lens or aplurality of lenses.

To avoid the rise in the temperature of the microfilm F, at least one ofthe first lens unit C1 and the second lens unit C2 constituting thecondenser lens system C may be made of heat-absorbing glass.

Although the above embodiment has a straight optical path extending fromthe light source assembly 1 to the microfilm F, the optical path can befolded using a mirror. The rise in the temperature of the microfilm isavoidable also in this case if the mirror used has heat-transmittingcharacteristics.

The filament 4 in the light source assembly 1 may be disposed in thevicinity of the position of first focus of the ellipsoidal reflector 3for the following reason. When the filament 4 is considered to be apoint light source, the light emanating from the filament 4 disposed atthe position of first focus will be concentrated on the position ofsecond focus 5, whereas the filament 4 actually has a definite size andtherefore forms an image of secondary light source of definite size atthe position of second focus. With this taken into consideration, thecenter of the filament 4 need not strictly coincide with the position offirst focus. Further the ellipsoidal reflector 3 may be one having aperfectly ellisoidal reflecting surface, or one having a reflectingsurface which is basically an ellipsoidal surface and formed by acollection of minute planes as shown in FIG. 1(B).

TABLE 1 below shows a specific example of the present embodiment. Withthis example, the light source assembly 1 focuses light at a position ata distance of 45 mm from the plane of opening of the ellipsoidalreflector 3. The lens back based on the microfilm surface (pupilposition of the projection lens based on the film surface) is 75˜71corresponding to the distance between principal points. The first lensunit C1 has two double concave lenses, the second lens unit C2 is adouble convex lens, and the third lens unit C3 is a Fresnel lens havingits Fresnel surface directed toward the light source assembly.

TABLE 2 shows more specific numeral values of the example given in TABLE1 and having a paraxial arrangement. Further, G1 and G2 are lenses ofwhich the first lens unit C1 are composed. G1 represents the lens nearerthe light source assembly 1 and G2 represents the lens nearer the secondlens unit.

With reference to TABLE 2, the second lens unit C2 is made ofheat-absorbing glass, and the third lens unit C3 of acrylic resin. Theradius of curvature of Fresnel surface marked with an asterisk refers toa coefficient a0 included in the following equation (B) which definesthe configuration of the Fresnel surface as shown in FIG. 5.

    R(h)=a0+a1h+a2h.sup.2 +a3h.sup.3                           (B)

wherein in a section containing the optical axis (FIG. 5), R(h)represents the distance between a point S at a height h from the opticalaxis 0 and the point of intersection, T, of the optical axis with astraight line perpendicular to the Fresnel surface and passing throughthe point S. When the coefficients a1, a2, a3 of the equation (B) aredeter mined for the projection lens to be used, a more suitableillumination system can be obtained.

                  TABLE 1                                                         ______________________________________                                                                     Distance                                                    Refracting                                                                             Outside  between                                                     power    diameter principal point                                  ______________________________________                                        Light source assembly                                                                      --         45                                                    (opening of reflector)                                                                                         69.9 to 34.9                                 First lens unit C1                                                                         -0.034114  35                                                                                     13.8 to 48.8                                 Second lens unit C2                                                                        0.009432   55                                                                                     114.5                                        Third lens unit C3                                                                         0.016667   80                                                                                     22                                           Microfilm surface                                                                          --                                                               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                       Radius of         Refrac-                                                     cur-    Axial     tive                                                        vature  distance  index                                        ______________________________________                                        Light source assembly    --                                                   (opening of reflector)                                                                                         64.5 to 29.5                                                          -60                                                                     G1            3       1.472                                                         60                                                   First lens unit C1               7.5                                                                   -60                                                                     G2            3       1.472                                                         60                                                                                     5 to 40                                                              110                                                  Second lens unit C2              10      1.527                                                         -110                                                                                  111.2                                                                 *29.484                                              Third lens unit C3               2       1.491                                                         ∞                                                                               23                                           Microfilm surface        --                                                   ______________________________________                                    

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the present invention, they should be construedas being included therein.

What is claimed is:
 1. An illumination system for use in an imageprojection apparatus, the illumination system illuminating an originalwhich retains an image to be projected, the image projection apparatushaving a projection lens unit in an exchangeable manner to project theimage of said original onto an image receiving plane with a differentmagnification, the illumination system comprising:a light sourceassembly which emits convergent light to illuminate the original; and anillumination lens group including, as arranged from the light sourceassembly side, a first lens unit of a negative power which is movablealong the optical axis, a second lens unit of a positive power and athird lens unit of a positive power, wherein the illumination lens groupfulfills the following equation,

    φ1+φ2<0

where φ1 and φ2 represent the powers of said first and second lensunits, respectively.
 2. An illumination system as claimed in claim 1,wherein said light source assembly comprises a lamp and a concavereflector behind the lamp.
 3. An illumination system as claimed in claim1, wherein said third lens unit comprises a Fresnel lens.
 4. Anillumination system as claimed in claim 1, wherein said first lens unitsincludes two lens elements each having a negative power.
 5. Anillumination system for use in an image projection apparatus,comprising:a light source assembly which emits convergent light; a firstlens unit of a negative power which is located in the vicinity of saidlight source assembly and movable along the optical axis; a second lensunit of a positive power which is positioned at the side opposite tosaid light source assembly with respect to said first lens unit; and athird lens unit of a positive power which is located at the sideopposite to said first lens unit with respect to said second lens unit;wherein the illumination system fulfills the following equation,

    φ1+φ2<0

where φ1 and φ2 represent the powers of said first and second lensunits, respectively.
 6. An illumination system as claimed in claim 5,wherein said light source assembly comprises a lamp and a concavereflector behind the lamp.
 7. An illumination system as claimed in claim5, wherein said third lens unit comprises a Fresnel lens.
 8. Anillumination system as claimed in claim 5, wherein said first lens unitsincludes two lens elements each having a negative power.